The present invention relates to a discharge lamp and a lamp unit. In particular, the present invention relates to a discharge lamp and a lamp unit used as a light source for projectors using a digital micromirror device (DMD) or a light source for a liquid crystal projector. The present invention also relates to an image display apparatus including such a discharge lamp or discharge unit.
In recent years, an image projection apparatus such as a projector using a DMD (digital light processing (DLP) projector) or a liquid crystal projector has been widely used as a system for realizing large-scale screen images. A high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. In the image projection apparatus, light is required to be concentrated on an imaging device (DMD panel or liquid crystal panel or the like) included in the optical system of the projector, so that in addition to high intensity, it is also necessary to achieve a light source close to a point light source. Therefore, among high-pressure discharge lamps, a short arc ultra high pressure mercury lamp that is close to a point light and has a high intensity has been noted widely as a promising light source.
Referring to FIG. 5, a conventional short arc ultra high pressure mercury lamp 1000 will be described. FIG. 5 is a schematic top view of an ultra high pressure mercury lamp 1000. The lamp 1000 includes a substantially spherical luminous bulb 110 made of quartz glass, and a pair of sealing portions (seal portions) 120 and 120xe2x80x2 made of also quartz glass and connected to the luminous bulb 110.
A discharge space 115 is inside the luminous bulb 110. A mercury 118 in an amount of the enclosed mercury of, for example, 150 to 250 mg/cm3 as a luminous material, a rare gas (e.g., argon with several tens kPa) and a small amount of halogen are enclosed in the discharge space 115. A pair of tungsten electrodes (W electrode) 112 and 112xe2x80x2 are opposed with a certain distance (e.g., about 1.5 mm) in the discharge space 115. Each of the W electrodes 112 and 112xe2x80x2 includes an electrode axis (W rod) 116 and a coil 114 wound around the head of the electrode axis 116. The coil 114 has a function to reduce the temperature at the head of the electrode.
The electrode axis 116 of the electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, and the W electrode 112 and the Mo foil 124 are electrically connected by a welded portion 117 where the electrode axis 116 and the Mo foil 124 are welded. The sealing portion 120 includes a glass portion 122 extended from the luminous bulb 110 and the Mo foil 124. The glass portion 122 and the Mo foil 124 are attached tightly so that the airtightness in the discharge space 115 in the luminous bulb 110 is maintained. In other words, the sealing portion 120 is sealed by attaching the Mo foil 124 and the glass portion 122 tightly for foil-sealing. Both of the sealing portions 120 have a circlar cross section, and the rectangular Mo foil 124 is disposed in the center of the inside of the sealing portion 120.
The Mo foil 124 of the sealing portion 120 includes an external lead (Mo rod) 130 made of molybdenum on the side opposite to the side on which the welded portion 117 is positioned. The Mo foil 124 and the external lead 130 are welded with each other so that the Mo foil 124 and the external lead 130 are electrically connected at a welded portion 132. The structures of the W electrode 112xe2x80x2 and sealing 120xe2x80x2 are the same as those of the W electrode 112 and sealing 120, so that description thereof will be omitted.
Next, the operational principle of the lamp 1000 will be described. When a start voltage is applied to the W electrodes 112 and 112xe2x80x2 via the external leads 130 and the Mo foils 124, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space 115 of the luminous bulb 110, and thus the mercury 118 is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes 112 and 112xe2x80x2. As the pressure of the mercury vapor of the lamp 1000 is higher, the emission efficiency is higher, so that the higher pressure of the mercury vapor is suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 25 MPa.
As shown in FIG. 6, the lamp 1000 can be formed into a lamp unit 1200 in combination with a reflecting mirror 60. FIG. 6 is a schematic cross-sectional view of the lamp unit 1200. The lamp unit 1200 can be used as a light source of DLP projectors or liquid crystal projectors, for example.
The lamp unit 1200 includes the discharge lamp 1000 and the reflecting mirror 60 for reflecting light emitted from the discharge lamp 1000, and the light emitted from the discharge lamp 1000 is reflected at the reflecting mirror 60 and emits in the emission direction 50. The reflecting mirror 60 has a front opening 60a on the side of the emission direction 50. A front glass (not shown) is to be attached at the front opening 60a for the purpose of preventing scattering at the time of lamp breakage. A lead wire 65 is electrically connected to the external lead 130 of the sealing portion 120 positioned on the front opening 60a side. The lead wire 65 is extended to the outside of the reflecting mirror 60 through an opening 62 for lead wire of the reflecting mirror 60. The lamp base 55 is attached to the other sealing portion 120xe2x80x2 of the discharge lamp 1000, and the sealing portion 120xe2x80x2 attached with the lamp base 55 is attached to the reflecting mirror 60.
The front glass is provided at the front opening 60a of the reflecting mirror 60, so that lamp unit 1200 is of an airtight structure. Therefore, when the lamp 1000 is heated, the temperature in the lamp unit 1200 becomes very high. Accordingly, the lamp 1000 is designed and produced with an estimation of the temperature of the lamp 1000 in the lamp unit 1200 to guarantee the lamp operation.
However, the inventors of the present invention found that when the conventional lamp unit 1200 is used as the light source of a DLP projector, the temperature of the welded portion 132 of the sealing portion 120 positioned on the emission direction 50 side became higher than estimated, and the welded portion 132 is oxidized and the lamp 1000 stops operating. In other words, molybdenum constituting the external lead 130 and the Mo foil 124 has the property of being oxidized at a temperature over 350xc2x0 C., and in addition, the molybdenum portion is positioned in the end of the sealing portion 120 and is in contact with ambient air. Therefore, when the temperature of the welded portion 132 that is more likely to be heated than other portions because of the contact resistance is increased to about 350xc2x0 C. or more, the welded portion 132 is oxidized (oxidization of molybdenum), and as a result, the conductivity of the welded portion 132 is lost, so that the lamp 1000 stops operating.
When the inventors of the present invention made research on what causes the temperature of the welded portion 132 to be higher than the temperature estimated at the time of design, they found that as shown in FIG. 7, reflected light 52 from an optical system 90 of a DLP projector disposed forward in the emission direction 50 of the lamp unit 1200 is incident to the reflecting mirror 60 of the lamp unit 1200, and the welded portion 132 of the sealing portion 120 positioned on the emission direction 50 side is irradiated with the reflected light 52. For example, in the case of a single panel DLP projector, the optical system 90 includes a color foil 70 of three primary colors (R, G, and B) disposed forward in the emission direction 50 of the lamp unit 1200 and a DMD panel 80 (constituted by a plurality of DMDs 82) for reflecting light that has passed through the color foil 70. The emitted light 51 from the lamp unit 1200 passes through the color foil 70 rotating at a rotation speed of, for example, 120 rotations per second, and becomes, for example, a red (R) light 54, which is projected on the DMD panel 80 via a condensing lens (not shown). In this case, the light of the emitted light 51 from the lamp unit 1200 that has not passed through the color foil 70 is incident again to the reflecting mirror 60 of the lamp unit 1200 as the reflected light 52 from the color foil 70.
The reflected light 52 incident to the reflecting mirror 60 is reflected at the reflecting mirror 60, and as shown in FIG. 8A, the welded portion 132 of the sealing portion 120 on the emission direction 50 side is irradiated with reflected light 53 from the reflecting mirror 60. Thus, because of the influence of the light 53 irradiating the welded portion 132, the temperature of the welded portion 132 of the sealing portion 120 becomes higher than the estimated temperature of the lamp unit alone without being in combination with the optical system 90. For example, the temperature may be about 50xc2x0 C. higher than the temperature estimated at the time of design.
Under the circumstances that a light source having a high intensity is in demand to improve the performance of DLP projectors, it is not desirable to reduce the output of the discharge lamp 100 (reduce the intensity) for the purpose of restricting the temperature of the welded portion 132 to not more than about 350xc2x0 C. during lamp operation. Furthermore, in light of the properties of molybdenum, it is difficult to achieve the welded portion 132 that is not oxidized at a temperature over about 350xc2x0 C.
Furthermore, the inventors of the present invention found that in operation in the structure shown in FIG. 7, the temperature of the sealing portion 120 is not uniformly increased, but the temperature of certain portions of the sealing portion 120 (e.g., a portion A where the welded portion 132 is sealed) is locally increased, as shown in FIG. 8B. In other words, they found that the sealing portion 120 is not uniformly irradiated with the reflected light 53 from the reflecting mirror 60, and a region (temperature focus region) 45 in which the temperature of the sealing portion 120 is a maximum is formed. Therefore, in the case where the welded portion 132 is positioned in the temperature focus region 45, the temperature of the welded portion 132 is even higher than the temperature estimated with the lamp unit alone.
Therefore, with the foregoing in mind, it is a main object of the present invention to provide a discharge lamp and a lamp unit having improved reliability that is achieved by suppressing the temperature increase in the connection portion (welded portion) in the sealing portion.
A discharge lamp of the present invention includes a luminous bulb in which a luminous material is enclosed and a pair of electrodes are opposed to each other in the luminous bulb; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively. Each of the pair of metal foils has an external lead on a side opposite to a side electrically connected to a corresponding electrode of the pair of electrodes. At least one of the pair of sealing portions is provided with a reflective film on a surface of the sealing portion in a portion where a connection portion of the external lead and the metal foil is sealed, the reflective film containing a material having a reflectance larger than that of a material constituting the sealing portion.
It is preferable that the reflective film contains a material having a heat radiation rate larger than that of the material constituting the sealing portion.
In one embodiment of the present invention, the connection portion is a welded portion where the external lead formed of molybdenum is connected to the metal foil formed of molybdenum by welding.
A lamp unit of the present invention includes a discharge lamp and a reflecting mirror for reflecting light emitted from the discharge lamp. The discharge lamp includes a luminous bulb in which a luminous material is enclosed and a pair of electrodes are opposed to each other in the luminous bulb; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively. Each of the pair of metal foils has an external lead on a side opposite to a side electrically connected to a corresponding electrode of the pair of electrodes. One of the pair of sealing portions is disposed on an emission direction side in the reflecting mirror. The one sealing portion disposed on the emission direction side is provided with a reflective film on a surface of the sealing portion in a portion where a connection portion of the external lead and the metal foil is sealed, the reflective film containing a material having a reflectance larger than that of a material constituting the sealing portion. The reflective film reflects light incident to the reflecting mirror from an optical system disposed forward in the emission direction and irradiating the connection portion, thereby suppressing a temperature increase in the connection portion.
It is preferable that the reflective film contains a material having a heat radiation rate larger than that of the material constituting the sealing portion.
Another lamp unit of the present invention includes a discharge lamp and a reflecting mirror for reflecting light emitted from the discharge lamp. The discharge lamp includes a luminous bulb in which a luminous material is enclosed and a pair of electrodes are opposed in the luminous bulb; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively. Each of the pair of metal foils has an external lead on a side opposite to a side electrically connected to a corresponding electrode of the pair of electrodes. One of the pair of sealing portions is disposed on an emission direction side in the reflecting mirror. The one sealing portion disposed on the emission direction side includes a temperature focus region where a temperature of the sealing portion is a maximum that occurs because of light incident to the reflecting mirror from an optical system disposed forward in the emission direction and irradiating the connection portion. The connection portion in the one sealing portion where the external lead and the metal foil are connected is provided in a position outside the temperature focus region, thereby suppressing a temperature increase in the connection portion.
In one embodiment of the present invention, the connection portion is a welded portion where the external lead formed of molybdenum is connected to the metal foil formed of molybdenum by welding.
In one embodiment of the present invention, the optical system comprises a reflection type imaging device, and a color foil for projecting emitted light from the reflecting mirror on the reflection type imaging device, and light irradiating the connection portion includes at least light that is a part of light emitted from the reflecting mirror toward the optical system, and is reflected by the color foil and incident to the reflecting mirror.
An image display apparatus of the present invention includes the above-described lamp unit, and an optical system using the lamp unit as a light source.
In one embodiment of the present invention, the optical system includes a digital micromirror device.
In the discharge lamp of the present invention, a reflective film is formed on the surface of a portion where a connection portion of the sealing portion is sealed. Therefore, light irradiating the connection portion can be reflected by the reflective film, and thus the temperature increase in the connection portion can be suppressed. In the case where the reflective film includes a material having a large heat radiation rate, the radiation of the reflective film also can suppress the temperature increase in the connection portion.
Furthermore, when such a discharge lamp and a reflecting mirror is combined, light that is incident to the reflecting mirror from the optical system disposed forward in the emission direction and irradiates the connection portion can be reflected by the reflective film. As a result, a lamp unit in which the temperature increase in the connection portion is suppressed can be provided. Furthermore, in another lamp unit of the present invention, the connection portion is provided in a position outside the temperature focus region of the sealing portion, so that the temperature increase in the connection portion can be suppressed, compared with the case where the connection portion is provided within the temperature focus region. The connection portion is, for example, the welded portion where the external lead formed of molybdenum and the metal foil formed of molybdenum are connected by welding. In the case where the optical system has a reflection type imaging device and a color foil, light irradiating the connection portion includes at least light reflected by the color foil and incident to the reflecting mirror. Furthermore, an image display apparatus can be provided by using such a lamp unit as the light source and combining an optical system (e.g., an optical system including a DMD as a component) therewith.
The present invention can provide a discharge lamp and a lamp unit having improved reliability that is achieved by suppressing the temperature increase in the connection portion in the sealing portion. Furthermore, it is possible to provide an image display apparatus by combining such a lamp unit and an optical system.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.